1. Field of the Invention
This invention relates to continuous direct casting of metal strip by a process employing a moving chill surface upon which molten metal is flowed for solidication beginning on the chill surface and progressing in a direction outwardly to the free top surface of the strip formed.
2. Prior Art
The advantages to be achieved in direct casting of molten metal into thin strip or sheet (hereinafter strip) on a continuous basis have long been recognized and numerous processes and apparatus have been proposed for the direct casting of metal strip. It is not believed, however, that any of these prior art processes or devices have been successfully used for use on a commercial basis, particularly for the production of a high quality strip suitable for use in the as-cast condition for the production of commercial products, or for further processing as by rolling or shaping by other means.
In the known prior art continuous or direct strip casting processes employing a continuously driven chill surface which contacts a melt of the metal to be cast, the melt is solidified by extracting heat through the chill surface so that a thin skin of the metal is solidified immediately upon contact with the chill. This skin grows in thickness as the chill moves progressively through or past the melt until the strip is completely formed. The thin skin initially formed is firmly adhered to the chill and this intimate bonded contact results in a maximum heat transfer from the melt to the chill.
As the solidifying skin progressively increases in thickness, the extraction of heat results in contraction of the solidifying strip at its bonded interface with the chill until the bond is broken, thereby resulting in a substantial reduction in the rate of heat extraction. The successful production of quality cast strip by such a process depends to a large degree upon the ability to extract heat at a uniform rate across the width of the strip to produce a uniform release of the cast product from the chill. One process for producing the uniform heat extraction through the chill surface is disclosed and claimed in copending U.S. patent application Ser. No. 885,718, assigned to the assignee of the present application, and involves establishing a natural oxide layer on the chill surface to provide an interface between the layer of molten metal and the chill surface and maintaining the natural oxide interface in a smooth layer of substantially uniform thickness.
In the production of directly cast strip on a chill, surface defects frequently occur in the top surface of the strip, i.e., the surface away from the chill. These defects may include surface cracks as well as variations in gauge or longitudinal shape of the strip and variations in transverse profile, making subsequent rolling or other processing of the cast strip difficult or impossible. Attempts have been made to control profile and gauge variations by contacting the top surface of the strip with a second chill roll prior to solidification of the top surface whereby the top roll cools and solidifies the top surface or skin of the strip and acts as a flow controller to limit the amount of molten metal deposited upon the chill. It is also known to contact the top surface with a pressure roll immediately after solidification of the strip in an attempt to control the strip gauge by a hot rolling action. Such pressure rolls have been internally cooled or otherwise configured to dissipate heat and provide a chilling effect on the strip.
U.S. Pat. No. 2,348,178 to J. M. Merle discloses, at FIGS. 1 and 4, means for flowing molten metal through a nozzle onto a continuous moving belt chill surface and a continuous cylindrical wheel chill surface, respectively. FIGS. 2, 8 and 9 of this patent disclose rolls 28 and 106 contacting the top surface of a stream of molten metal issuing from a nozzle in the bottom of a tundish, with the roll acting as a metering valve to control the size of the nozzle opening and hence the rate of flow of molten metal onto the chill surface. FIGS. 5 and 6 of this patent disclose the use of rolls 36 and 36a which act on the top surface of the formed, or solidified strip. All the rolls contacting the top surface, i.e., rolls 28, 36, 36a and 106 are preferably internally cooled as by flowing water therethrough so that the rolls aid in solidification of the molten metal. As indicated above, it is not believed that either the apparatus or process of the Merle patent have been successfully used for the production of directly cast strip on a commercial basis.
Japanese published patent application No. 1983-41656 (published Mar. 10, 1984) discloses apparatus for directly casting thin metal strip in which molten metal is supplied from a suitable source such as a ladle and maintained at a uniform level in an open tundish having one wall defined by a large cylindrical casting wheel which is internally cooled by a circulating cooling fluid. The casting wheel is rotated in a direction to move its surface in contact with the molten metal in the tundish upwardly while heat is extracted through the chill surface to progressively form a strip 9 having a thickness "T" as the strip emerges from the top of the molten metal pool in the tundish. A water cooled top roll 10 is positioned to engage the top surface of the solidified strip and reduce the strip to a thickness "t" by a hot rolling action. Liquid sprayed on the emerging strip assures a solidified top surface before contact with the top roll.
Published European patent application No. 198,669 (published Oct. 22, 1986) discloses apparatus for directly casting thin metal sheet in a continuous process employing a large chill roll positioned to have its outer surface contact molten metal in a tundish and driven to withdraw the solidifying strip adhered to its surface. A water cooled top roll is positioned to extend into the molten metal supply in the tundish and is driven at the same peripheral speed as the large chill roll but in the opposite direction so that molten metal is fed between the two rolls to emerge in a thin solid sheet which is then peeled from the large roll surface. A similar apparatus and process is disclosed in Japanese Patent 59-13551.
Difficulty has been encountered in attempting to use a top roll of the type disclosed in the prior art in contact with the formed or forming strip moving on a primary chill surface. For example, when a cooled top roll is employed in contact with molten metal as disclosed in the above-mentioned Japanese patent and the European patent application, there is a tendency for the top surface to solidify on contact with the chilled surface and to adhere to the surface in the same manner as the bottom strip surface adheres to the primary chill. Although this bond between the top strip surface and the cooled top roll may be only brief and broken more readily as a result of the relatively small radius of the top roll, cast sheet defects nevertheless result.
Attempts to form directly cast thin metal strip using a top roll engaging and rolling the top surface of the strip after solidification also have not proved successful. Although the just-formed and solidified strip is still extremely hot and soft when contacted with a top shaping roll, metal movement in such a rolling operation can essentially only be in the rolling direction, i.e., longitudinally of the formed strip, and therefore corrections of the transverse profile of the strip cannot be accomplished. Further, surface defects in the form of minute cracks may be formed prior to reaching the rolling operation, and rolling can result in propagating such cracks rather than eliminating them.
Attempts to shape the strip by a hot rolling operation immediately following solidification of the strip on the chill also results in forcing of the bottom surface of the soft strip into any grooves or other surface depressions employed on the chill surface. This not only produces an undesired surface configuration on the strip but also results in uneven and difficult release of the strip from the chill surface.
It is, accordingly, a primary object of the present invention to provide a novel process of and apparatus for producing directly cast thin metal strip in a high speed commercial operation.
Another object is to provide such a process and apparatus to produce a directly cast thin metal strip having an improved transverse profile and more uniform longitudinal shape.
Another object of the invention is to provide such a process and apparatus having a smooth uniform top surface substantially free from transverse cracks.